Field of the Invention
Embodiments of the invention relate to an active matrix organic light emitting diode (OLED) display and a method for controlling a luminance thereof.
Discussion of the Related Art
An active matrix organic light emitting diode (OLED) display includes organic light emitting diodes (OLEDs) capable of emitting light by itself and has advantages of a fast response time, a high emission efficiency, a high luminance, a wide viewing angle, and the like.
The OLED serving as a self-emitting element includes an anode, a cathode, and an organic compound layer formed between the anode and the cathode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode and the cathode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML and form excitons. As a result, the emission layer EML generates visible light.
The OLED display arranges pixels each including the OLED in a matrix form and adjusts a luminance of the pixels depending on grayscale of video data. Each pixel may include a driving thin film transistor (TFT) controlling a driving current flowing in the OLED depending on a gate-to-source voltage of the driving TFT, a capacitor keeping the gate-to-source voltage of the driving TFT constant during one frame, and at least one switching TFT programming the gate-to-source voltage of the driving TFT in response to a gate signal. A luminance of the pixel is proportional to a magnitude of the driving current flowing in the OLED.
In recent years, as the OLED display is applied to a mobile device or a wearable smart device, etc., various attempts have been made to improve outdoor visibility.
To improve the outdoor visibility, as shown in FIG. 1, a method for adjusting a gamma curve to modulate input video data has been known. Because a brightness of peripheral light in an outdoor environment is generally greater than a brightness of peripheral light in an indoor environment, the OLED display must be able to implement a high luminance so as to improve the outdoor visibility. However, because the highest gray level is fixed to a specific luminance value in the method for modulating the input video data, it is difficult to implement a higher luminance than the specific luminance of the highest gray level. Further, the detailed representation is reduced due to a loss of gray scale, and so-called dot noise may be generated at low gray levels.
A method for increasing an uppermost gamma reference voltage generated from an internal gamma string of a data driver to implement the high luminance has been known. However, an increase in the uppermost gamma reference voltage increases a range of a data voltage generated by dividing the uppermost gamma reference voltage. This leads to an increase in a swing width of the data voltage output from each channel of the data driver. When the output swing width of the data voltage increases, power consumption of the data driver increases, and an output buffer of the data driver has to be re-designed as a high voltage element. Hence, the size of the data driver needs to be increased. Thus, it is difficult to apply the above-described methods for implementing the high luminance to the mobile device and the wearable smart device, which have a small size and small battery capacity.